1. Field of the Invention
This invention relates to a food package useful for microwave heating applications, and particularly to a container or tray having a sealed closure which becomes easier to open upon microwave heating.
2. Description of the Related Art
There has been much interest recently in food packaging materials for foods cooked in a microwave oven. U.S. Pat. No. 4,267,420, to Brastad, discloses a food product wrapped with plastic film having a very thin microwave interactive coating. The film conforms to a substantial portion of the food product. The coating converts some of the microwave energy into heat which is transmitted directly to the food surface so that a browning and/or crisping is achieved.
U.S. Pat. No. 4,676,857, to Scharr, discloses a microwave heating material and method for its preparation. A preselected metallized pattern, such as dots, spirals, or circles, is disposed on at least a portion of a dielectric material. The dielectric material may be in the form of a flexible wrap.
Other inventions have used the fact that various polymeric materials lose strength at elevated temperatures to perform useful packaging functions. U.S. Pat. No. 4,404,241, to Meuller et al., discloses a microwave package with a means for venting vapor. The vent is in the form of an aperture in the multilayer sheet which forms the package, and is covered with a continuous sealing layer of an extrudable hot melt material. When this material is subjected to slight pressure in combination with heat, softening and flow occurs at temperatures effective to permit venting of steam or other vapor without sufficient pressure build-up to distort the package.
U.S. Pat. No. 4,561,337, to Cage et al., discloses a bag containing a mixture of edible popcorn ingredients suitable for use in microwave ovens. Portions of the panels of the bag contain a coating that is sensitive to pressure and heat, forming a seal along the top edge of the panels. The seal has sufficient strength to withstand the internal steam pressure generated by the moisture content of the kernels for at least one-half of the popping process. Preferably, the bag will vent at the top seam before the process is completed to allow steam to escape.
Food packages sealed using conventional techniques, such as heat sealing a lid to a flange surrounding the opening of a tray using a synthetic resin as the adhesive, can be difficult to open along the seal. A consequence of this construction is that a significant tearing force is required to break the seal and thus the package must be fabricated using material of a sufficient thickness to resist deformation during opening by the consumer. In part, such seals are employed to ensure that the food remains securely sealed during the manufacturing and handling steps which are performed after the food product has been placed in the package and sealed. One manufacturing step which may be performed after sealing the package is heat pasteurization or heat sterilization. Unfortunately, containers securely sealed in this fashion present difficulties for consumers in opening the container after microwave heating. This is especially troublesome when the food product is, at least in part, in liquid form, for example as is encountered in soups, stews and products with gravy. Opening the sealed containers of the prior art often result in spillage of the liquid food product.
Some food containers for microwave applications are designed, for example with perforated lids, to avoid this problem and their lids or closures are at least partially removed prior to microwave heating, in order, inter alia to permit steam generated during microwave heating to be vented and to facilitate further opening after microwave heating. Unfortunately, with these containers there is a problem with spillage prior to heating, and product loss during heating, e.g. by splattering, is also a problem.
The prior art also has followed other approaches. U.S. Pat. No. 4,605,142 to Itoh et al., for example, describes a package or container having a continuous ridge extending along a flange at the opening of a tray. The ridge has a projection extending toward the outer periphery of one or both sides of the tray. The ridge has the effect of reducing the area of the seal. By reducing the total area (width) of the seal in combination with the projection, the force required to initiate opening and to propagate the opening of the closure is purportedly reduced.
U.S. Pat. No. 3,217,871 describes using an adhesive for sealing a package, which remains partially non-adherent through the sealing operation. In one embodiment, one of the opposing sealing surfaces has a discontinuous adhesive coating, while the other surface has a continuous adhesive coating. A problem with this approach is premature opening of the seal.
The present invention, in contrast, provides a sealed food package or container for use in a microwave oven which is sealed securely during packaging and which remains securely sealed during conventional heat processing operations and subsequent handling. A novel feature of the package is that the seal becomes more easy to open upon heating in a microwave oven so as to facilitate opening of the container by the consumer. The present invention further provides a package which permits venting of steam generated in the package such as by the food product during heating.